Cam profile mania

[Cuttings]

Ive been away for a few days so thought I should stop by to see how this discussion was going. Very good I would say.
I have something else to throw in the mix. Most of you probably remember the screw in little mufflers that Briggs and Stratten used to use on their 3HP engines that were used on almost everything from lawn mowers to rototillers etc. I had a lawn mower with one of those and the muffler was getting burnt out. The engine was starting to pop and fart quit a bit and was starting to get a bit loud so for about $5 in those days I bought a new muffler to stick on it. Immediately it ran perfectly again. I guess it needed a little exhaust back pressure to run properly. I never did take the time to think it out to figure out why, but used the same thing a few times after that when they started to run poorly.

[A.K. Boomer]

You bet and good observation, engine cam profiles and duration engineered with flow dynamics that have a factory exhaust system with copious amounts of restriction can fall on their face if this restriction is removed and of course same goes for ones with huge flow then getting restricted...

On a single cylinder you don't have the advantage of another cylinder providing scavenging for you at a collector unit down the road but they can still benefit from this effect with the length of free flowing pipe before the unit goes into a cat or muffler or some other restrictive unit,
length of pipes before collectors are VERY critical on where you want your "pipe" to come on - as in start to pull good, and keeping the pipes all equal lengths is critical too - hence the name "tuned headers" resulting in mild bends here and there to eat up space and make the shorter runs as long as the long ones, also makes for some funky looking pipes to people who don't know the reason "Why"

[The Artful Bodger]

Exhaust flow is worthy of careful study in a hit and miss engine. Not only is the power effected by exhaust dynamics but perhaps more significant is the exhaust flow during the miss cycle where friction in the exhaust flow contributes to the slowing of the engine between power cycles.

[Cuttings]

I would think this is quit critical on the Briggs, because when you remove the muffler from the engine the exhaust valve is right there.
No muffler or a burnt out one would cause no restriction at all .
Brian - I was wondering how much trouble it would be to try a little muffler on your hit and miss engine?

[brian Rupnow]

Cuttings--I took a little muffler off to put the vertical pipe on.

[Cuttings]

Sorry, I guess my old memory had erased that bit of information.

[gbritnell]

A hit and miss engine is a very rudimentary internal combustion engine. It came in sizes ranging from a couple of horsepower to upwards of 100 or more. They were used to power everything from ice cream makers to oil field pumps.
Although the production lasted into the 40's they still kept the same mechanical exhaust, atmospheric intake configuration. The reason being this mechanism was used to control the engine speed. At the time there were many engines using cam controlled intake and exhaust breathing but these engines had some type of throttle control.
Ok so you say that you could still use this type of control on a mechanical valve engine, yes but what would happen. With the hit and miss control being used on the atmospheric intake engine the intake valve won't open until the exhaust closes and the piston creates a negative enough pressure in the cylinder to draw the valve open. If it were used on a mechanically operated intake valve engine when the engine went into it's coast or non-power cycle the intake would be opening and closing and therefore would draw fuel into the cylinder. Mind you not as much as if the exhaust were closed but enough over several coast cycles to flood the cylinder. Now when the exhaust valve was released the engine would have to fire all this extra fuel, maybe yes but quite possibly no. You have to consider the fuel and ignition systems of the era.
The simplicity of the system is quite ingenious not counting the fact that a different cam would be needed along with an extra pushrod and guide and more space required to install them.
There had to be some merit to the design because of it's longevity and the vast number of engines produced by so many manufacturers.
These engines started out as basic power sources and that's all they ever were. In most cases they were quite reliable and very tolerant to the environment they worked in considering they had no air filters, were hand lubricated and lived in dirty conditions. They were truly inefficient as far as internal combustion engines go so I'm sure there wasn't much thought about exhaust characteristics and flow numbers. I can't think that some engineer at John Deere in 1912 said to his boss “you know if we put a tuned muffler on this baby we'll really get some power out of it.”
Just my two cents worth.
gbritnell

[A.K. Boomer]

Well said GB - they served a purpose and did things better than other engines of the same era due to conservation of fuel,

sometimes the oil wells produced gas too and they would just run off of that so not that much of an issue - but other times fuel had to be brought in and the engines would run on either propane or gas, they were misers and if properly designed and tuned were tough to beat that way.

[J Tiers]

Yes, pretty much stone hammer simple. And they needed to be, because they were operated by farm folks to whom the engine was extreme high technology.

In other cases the motor would be operating out in an oil field for weeks without attention other than maybe some lube every so often. I recall seeing them still operating on "grasshopper" pumps when I was a kid.

It's not as if the state of motor development was so rudimentary, they were still being designed as hit and miss at a time when relatively sophisticated aircraft engines were being made, and Lindbergh flew to Paris using one of them*. So there were reasons for making the engines as they were made. Simplicity, ruggedness, easy repair, etc.

Now, the full scale engine may be significantly different from a model. Scaling down reduces masses by a cube law, but dimensions only linearly, so many things change relation. A perfectly scale model, or scale motor that is not a reproduction of a real motor, may need many things changed in order to operate well.

The start of the whole issue here was to get better slow speed hit and miss action, something not improved by the cube law reduction of mass. The flywheel dimensions may have to be larger to store enough energy to get over compression etc, and the engine as Kerzel designed it and Brian made it does have fat flywheels, as one would expect. If directly scaled from a full scale engine, the engine likely would have to operate much faster.

* as an example of general technological thought at the time, the field effect transistor was described and analyzed in 1929 by a Frenchman, but the means of making it physically was not developed until much later. And the jet engine as well as liquid fuel rockets were not far off. Not a dark age of technological backwardness in any way.

[BCRider]

A couple of thought occur to me on the timing.

If there's anything that is not good it would be running too short a timing that results in a late opening point and rounding BDC with a lot of pressure still in the cylinder. That's a real no-no in any discussions on engines I've read. It would both rob power and creates unnecessary load on the various bearing points. Even a valve that just starts opening right at BDC will see a lot of pressure still acting on the piston for some time after BDC until the valve opens up far enough for long enough to let the pressure out. These and more is why I've always seen any engine timing start opening the exhaust some amount before BDC is reached so that the cylinder pressure can start to leak down before BDC so there's less pressure robbing power away as the piston starts the upward exhaust stroke. And again even at the slow speeds we run on these model engines there's likely much to be said for starting to open at some angle up to perhaps as much as 20° before BDC to ensure that as the piston passes BDC there's only some residual pressure which is in the process of dropping fast.

On the timing for closing there's some issues to consider too. Again there is very little linear piston travel during the 10° of rotation either side of TDC. So there is nothing wrong with a cam timing that finishes closing the exhaust a few degrees after TDC. On the other hand a cam that closes the exhaust a little earlier than TDC will also be in the process of closing for quite a few degrees leading up to that point and producing a restriction in the flushing of the residual exhaust. So a cam that closes the valve right at or at all before TDC can result in some slight compression being present as the piston moves towards TDC. And any residual pressure will prevent the intake from opening until later in the intake stroke. In addition there's a slight braking action due to closing the exhaust too early so that some slight compression is created. So all in all I feel that even in a slower turning engine like this there's some advantage to an exhaust cam timing which finishes closes at something around 3 to 6° AFTER TDC.

If you like what you read then clearly the original Kerzel cam fails due to having only 168.4° of total duration. On the other hand the Webster cam with 234° of total duration "might" be a little longer than ideal for the Kerzel engine. If we add up the 180° of the exhaust stroke with, say, 5° of overlap at TDC and even 20° before BDC we only have 205° that my guts say would be good. And in fact if the Webster cam was set to close at 5° ATDC as I feel is a good thing then it would be starting to open the exhaust at only 131° ATDC on the power stroke. That's not bad but it's opening up a lot earlier than is really needed for slow speeds such as on a hit n'miss engine. And as a result it wastes about 1/10 of the power stroke give or take. (note that the last couple of sentances are heavily edited from the original which was based on bad math-BCR)

So it would seem that even by armchair analysis that neither of these cams is at all optimum. You've got the "too warm" Daddy Bear's profile on one hand and the "too cold" Mommy Bear's profile on the other. What you need from where I'm sitting is the in between Baby Bear's solution of something around 195° to 215° total duration which provides whatever dwell is created in the process. Where to fit it in this range? I feel like with a closing point of 5°ATDC that a 195° total duration would be a little tight with only the last 10° BBDC to leak off some exhaust. 205° would be better. And certainly a total duration of 215° would see the pressure all but bled off as the piston starts upward. Again this is my armchair engineer's view. But from where I'm sitting it seems like anything in this range would be better than either of the other two.

[BCRider]

I was just checking in to see if there were any replies and was reading over my post. My math on the opening point of the Webster profile was WAY off. For some odd reason I subtracted the 49° excess of the Webster from only 90° instead of 180°. So the 234° cam would not open at 41° as I posted but rather at 131° after TDC. So that's not bad at all and certainly not the sort of train wreck I made it out to be.

I've corrected my post above.

[The Artful Bodger]

I guess I was one of those farm folks as we had a hit and miss engine to drive our sheep shearing plant. It was quite a small engine and was very reliable perhaps because it was only run for three weeks each year.

Unfortunately I do not have access to a H&M engine nowadays but if I did I would rig up a camera and strobe and try to get an idea of when the inlet valve actually opens and does it always fully open.

[The Artful Bodger]

Thinking further (while wondering who has a H&M around here!) I am thinking that a solenoid operated inlet valve might make an interesting experiment.

The inlet valve could be opened at the very start of the induction stroke instead of waiting for cylinder depression to overcome the spring and open the valve which would stay open until the very last point of the induction pressure differential rather than have the spring start to close the valve before then.

[BCRider]

Thinking further (while wondering who has a H&M around here!) I am thinking that a solenoid operated inlet valve might make an interesting experiment.

The inlet valve could be opened at the very start of the induction stroke instead of waiting for cylinder depression to overcome the spring and open the valve which would stay open until the very last point of the induction pressure differential rather than have the spring start to close the valve before then.

Or we could run BOTH valves on solenoids and skip the cam altogether. Or we could do what was done on the follow on designs and use two CAM controlled valves.....

Part of the beauty of the vacuum inlet is that it only opens when the exhaust closes and a vacuum is produced. If a solenoid run valve opened each time instead of only when it was needed we'd get a lot of spitting back through the carb. So the solenoid would need its own cam sensor, interrupter and other "stuff". Simpler at that point to skip the solenoid, power source and other junk and just run the inlet off a second cam lobe.

In the end the H&M is what it is and unless we are interested in some updated Steam Punk variation is likely best kept simple... at least to my thinking.

Mind you a jazzed up Steam Punk variation to the whole design might be a LOT of fun!

[Lew Hartswick]

Thinking further (while wondering who has a H&M around here!) I am thinking that a solenoid operated inlet valve might make an interesting experiment.

The inlet valve could be opened at the very start of the induction stroke instead of waiting for cylinder depression to overcome the spring and open the valve which would stay open until the very last point of the induction pressure differential rather than have the spring start to close the valve before then.

I've been thinking this very thing for the last half doz. pages of this thread. NOTE! I'm not a model engine builder nor am I likely to ever be one. So just from the stand point of an observer, I think it would be an interesting thing for "Brian" to do. :-)
...lew...